Processes and Patterns of Glacial Erosion

Abstract

Subglacial observations are presented of three important erosional processes, abrasion of the bed by debris embedded in the glacier sole, crushing of subglacial bedrock due to the pressure fluctuation produced by glacier flow over a bedrock hummock, and removal by plucking of the debris produced by the previous processes. The processes are then analyzed theoretically.

It is suggested that for any one ice velocity and increasing effective normal pressure at the glacier bed, that abrasion rates will rise to a maximum, thereafter falling rapidly until a critical pressure is attained beyond which the bed is not abraded and lodgment till accumulates. Normal abrasive processes are considered sufficient to explain p-forms such as Sichelwannen, cavettos, and glacial grooves. The progressive abrasion of a bedrock hummock is simulated, and for low pressures, high ice velocities or small bed roughness roches moutonnees or valley steps form with a down-glacier asymmetry, for high pressures, low velocities or high bed roughness, rock-cored drumlins form with an up-glacier asymmetry.

Analysis suggests that at constant ice velocity crushing on the lee flanks of bedrock hummocks becomes increasingly important under increasing ice thickness, rising to a maximum and then falling off rapidly. It is suggested that the plucking mechanism, a necessary complement to abrasion and crushing in eroding a rock bed, will be co-extensive with these processes. Plucking alone is necessary to erode unlithified sediments.

The theory also explains the forms of glaciated U-shaped valleys, cirques, valley steps, over deepened basins and fjords.